The present invention relates generally to the field of transmitting digital information over a computer-to-computer radio data link. More specifically, the present invention relates to the Navy Technical Data System (NTDS) and to the radio link (Link-11) permitting and controlling intercommunication between the various NTDS computers in the network.
The Link-11 data message consists of 48 binary digits. These bits are assembled in two frames, each of 24 bits. Six Hamming code parity check bits are added in each frame to make 30 bits for transmission. Each of the 15 pairs of bits in the frame are converted into a phase shift, with shifts of -45.degree., -135.degree., -225.degree. and -315.degree. representing bit-pairs (1,1), (0, 1), (0, 0) and (1,0). Differential coherent phase shift modulation is applied to each of fifteen audio tones. Fourteen tones are at frequencies of 935 Hz through 2,365 Hz at 110 Hz intervals, with a fifteenth tone at 2,915 Hz. The transmitted audio signal is the sum of the fifteen modulated signals and a Doppler tone. Phase transitions occur at frame boundaries, but within each frame the tones may be considered continuous wave (CW) signals. The Doppler tone is a CW 605 Hz tone which is used for Doppler frequency shift corrections. The transmitter uses the audio signal to amplitude modulate an HF radio frequency carrier and transmits a double sideband suppressed carrier HF signal. Each of the two sidebands, the upper sideband (USB) and the lower sideband (LSB), contains the same audio signal on transmission, but anomalies in the propagation path, noise, and interference may cause the received sidebands to differ.
Each transmission, except in the network synchronization mode, begins with a preamble consisting of five frames followed by a phase reference frame. Only the 605 Hz Doppler correction tone and the 2,915 Hz framing tone are transmitted during the five preamble frames. The 2,915 Hz tone is given twice its normal amplitude and is phase shifted 180.degree. at each frame boundary to provide for frame synchronization. The 605 Hz tone is given twice the amplitude of the 2,915 Hz tone during the preamble frames and twice the amplitude of the information carrying tones for all other transmitted frames. The preamble is followed by a reference frame during which all data tones are transmitted to establish a reference phase for each.
The data rate is selected by the operator. It may be either a fast rate of 2,250 bps with a data frame interval of approximately 13.3 msec, or it may be a slow rate of 1,364 bps with a data frame interval of approximately 22 msec.
There are five modes of operation in a Link-11 net: Roll Call, Net Sync, Net Test, Broadcast, and Short Broadcast. This portion of the system operation is called Link Control.
Roll Call is the normal mode of operation. One unit is designated as the Net Control Station (NCS) and all other units are called Picket Stations. The net cycle is started at the NCS which calls the first Picket Station address. The NCS receives the data transmitted by the called Picket Station and calls each Picket Station in turn. When the last station in the net has been called, the NCS transmits its own data followed by a Picket address and the cycle repeats. The Picket Stations receive every transmission but transmit their own data only after recognizing their own station address. Each transmission sent by a participating unit is preceded by a message header as shown in FIG. 1, the numbers indicating the number of frames.
Normally the NCS transmits Net Sync prior to Roll Call operation. During Net Sync the NCS transmits a continuous sequence of preamble frames to permit Picket Stations to synchronize with the NCS. In the Net Test mode, the NCS transmits a predetermined data sequence which is received by the Picket Station data terminals and compared against the same internally generated sequence.
In Broadcast mode a station (either NCS or Picket) transmits until the data terminal is manually reset. In Short Broadcast mode a station (either NCS or Picket) initiates a single transmission sequence and the data terminal is reset at the end of that sequence.
The information is carried by each tone in the form of phase shifts from the same tone in the previous frame. Each tone represents two data bits. The remainder of the Link-11 functions will be described in conjunction with the description of the present invention.
The above functions have traditionally been implemented in special purpose hardwired equipment. The hardware currently in use is expensive, at least six times more costly than the programmable data terminal set (PDTS) of the present invention. The hardwired equipment is unique and difficult to maintain. Installation of the equipment is expensive due to its large size and the requirement for chilled water cooling. Changes to equipment are difficult to implement and costly.